They’re technically breeds of the same species, so yes, they’re compatible. What’s interesting about both forms of ninetales is actually that whether their young is fire or ice depends on two factors: which region the egg is laid in and whether or not one of the parents is holding an everstone. The more obvious of the two is the regional detail. An egg laid in Alola will always hatch into an ice-type, whereas an egg laid elsewhere will hatch into a fire-type. This may be because the unique environment of Alola triggers the shift towards vulpix’s Alolan variation, even if one of their parents is the fire-type ninetales.
As for the everstone, that is an interesting object in general when it comes to pokémon breeding. By itself, the everstone has the power to override a pokémon’s physiological shifts. Outside of breeding, this means that the everstone has the power to block evolution, even if a pokémon wants to evolve and/or is in contact with an evolution stone. When it comes to breeding, however, an everstone has the power to boost a parent’s genetic traits and “lock” them into their young. For example, let’s say you have a pikachu with Lightning Rod as its ability. Even though this is technically a recessive gene for the pikachu species, giving that parent an everstone overrides the egg’s tendency to shift towards the more dominant option of Static and instead locks it onto the same ability as the parent pikachu. Something very similar happens when breeding an Alolan and a standard variation of a pokémon. If you give an everstone to the Alolan parent, the breed of the egg will “lock” onto the genes for an Alolan variation. If, however, the standard variation is given the everstone, then the resulting egg will become a standard variation of that species.